You should also be aware that OpenGL lighting is a per-vertex operation. This means that the color of each vertex of a face will be computed based on how the light will affect that location, and then the color of the rest of the face is interpolated from those vertices.

So for example say you have a large quad, and the light shines in the center in such a way that none of the four vertices are affected by the light. In this example, you won't see any lighting effects at all.